Radio frequency (RF) power amplifier structures achieve high RF power by operating RF amplifier circuits in parallel, splitting one or more RF signals among the inputs to each of the parallel circuits. One such RF power amplifier structure is shown in FIG. 1. The RF power amplifier 100 comprises two RF amplifier stages 104, 106 configured to operate in parallel, a signal splitter 102, a signal combiner 128, and a phase-matching transmission line 126. Each RF amplifier stage 104, 106 typically includes a class AB RF amplifier 116, 122, an input matching circuit 114, 120, and an output matching circuit 118, 124.
The signal splitter 102 typically comprises a three decibel (dB) splitter that includes two 70.7 ohm, one-quarter wavelength transmission lines 108, 110 and a 100 ohm isolation resistor 112. The signal combiner 128 typically comprises a one-quarter wavelength transmission line transformer 130 having a nominal characteristic impedance of 70.7 ohms. The phase-matching transmission line 126 comprises a one-quarter wavelength transmission line having a nominal characteristic impedance of 50 ohms and is used to offset the one-quarter wavelength phase shift introduced by the signal combiner 128.
When an RF signal 134 is applied to the RF power amplifier 100, the signal splitter 102 divides the RF signal into two equi-amplitude RF signals. The signal splitter 102 also isolates the RF amplifier stages 104, 106 from each other via the isolation resistor 112. RF amplifier stage 104 applies a gain to one of the equi-amplitude RF signals to produce a first amplified RF signal. Similarly, RF amplifier stage 106 applies a gain to the other equi-amplitude RF signal to produce a second amplified RF signal. The RF gains applied by the RF amplifier stages 104, 106 are generally equal since the RF amplifiers 116, 122 are often the same active devices (e.g., RF power transistors).
The signal combiner 128 then combines the first and second amplified RF signals to produce a combined output signal. The phase-matching transmission line 126 introduces a one-quarter wavelength phase shift to the RF signal applied to RF amplifier stage 106 to compensate for the one-quarter wavelength phase shift introduced by the signal combiner 128 to the first amplified RF signal produced by RF amplifier stage 104, so that the first and second amplified RF signals will be in phase when combined.
A drawback to the RF amplifier structure depicted in FIG. 1 is that the use of a 3 dB signal splitter reduces the potential gain of the configuration. For example, suppose the RF amplifiers 116, 122 are power transistors that provide 10 dB of gain at the frequency of interest. Operating in parallel, the two transistors can produce a combined output signal of 100 watts (50 decibels above a milliwatt (dBm)). In order to produce 50 watts each, each transistor 116, 122 requires 5 watts (37 dBm) of input power since its gain is 10 dB. For the signal splitter 102 to provide each transistor 116, 122 with 5 watts of input power, the signal splitter 102 requires an input RF signal 134 of 10 watts (40 dBm), since a 3 dB splitter divides the input power equally between the two RF amplifier stages 104, 106. Thus, the gain of the RF amplifier structure in this example would be 10 dB (i.e., 10 watts (40 dBm) of input power to produce 100 watts (50 dBm) of output power).
As a further example, suppose that instead of two RF amplifier stages in parallel preceded by a signal splitter, there are four RF amplifier stages in parallel, each pair of stages preceded by a 3 dB signal splitter and the inputs to the two signal splitters coupled to the outputs of a third 3 dB signal splitter. For each of the two pairs of parallel amplifier stages to produce an output signal of 100 watts, or 200 watts when the outputs of both pairs are combined, each of the two signal splitters immediately preceding the amplifier stages requires 10 watts of input power. To provide 10 watts to each of the two signal splitters, the third 3 dB signal splitter requires 20 watts of input power. Again, the gain for the entire configuration of four parallel RF amplifier stages is 10 dB (i.e., 20 watts (43 dBm) input power to produce 200 watts (53 dBm) of output power).
If the signal splitter could be eliminated and more gain could be obtained out of the above exemplary RF amplifier configurations using the same RF power transistors, a number of design advantages and cost savings may be realized. One design advantage is that the RF power transistors would not have to be operated at their specified gain levels. By operating each transistor at less than the transistor's specified gain (e.g., by requiring less than 10 dB of gain per transistor), the design would be more tolerant to natural variations in gains between devices. Second, if the configurations had more overall gain, then they would require less input power from preceding RF amplifier stages. Reducing the power required from preceding RF amplifier stages could permit the use of lower power, less expensive RF power transistors in a preceding RF amplifier stage. Also, requiring less input power might allow for the elimination of one of several preceding RF amplification stages, thus eliminating the need for one or more RF power transistors altogether. Since the RF power transistors are among the most costly components in a high power transmitter, the cost savings could be significant.
Therefore, a need exists for a method and apparatus for amplifying RF signals that eliminates the need for a signal splitter and thereby increases the gain of an RF power amplifier configuration.